Suppress trigger execution when installing Flatpaks
    
    We don't need to run triggers to update caches, etc, since we're
    just installing Flatpaks to get an ostree that we copy to the
    destination, and in some execution environments (like a koji
    runroot), the bwrap call to run the trigger might fail, so
    export an empty FLATPAK_TRIGGERSDIR.
    
    Signed-off-by: Owen W. Taylor <otaylor@fishsoup.net>